1. Field of the Invention
The present invention pertains to communication systems and methods for communicating information from a source terminal to two or more destination terminals via an Integrated Services Digital Network (ISDN) network or other network. In particular, the present invention pertains to establishing over a source sub-link, e.g. an ISDN D channel, a communication link between a source terminal and multiple destination terminals via a modified a 1.times.N ("one by N") switch and multiple destination sub-links, such as separate ISDN B channels.
2. Discussion of the Background
Conventional facsimile devices communicate over the Public Switch Telephone Network (PSTN) using analog signals that are transmitted over conventional telephone lines. The source terminal (e.g., a facsimile device, computer with scanner and modem facilities, or another device that transmits and/or receives data) converts digital scanned information into a corresponding analog signal so the same may be sent over the PSTN telephone line, via a telephone switch facility, to the destination terminal. The source terminal receives the analog information and converts the analog information back into digital signals which form the basis of an image to be printed, perhaps on facsimile paper.
The Integrated Services Digital Network (ISDN) is emerging as a next generation worldwide public telecommunications network that will replace existing public switch telephone networks and provide a variety of services that are not offered by the PSTN. ISDN will allow the transmission of various types of data between various types of ISDN terminal equipment (TE).
A portion of the ISDN link between a source terminal and a central office, which has a switch facility, is referred to as a "digital pipe". A capacity of the pipe is generally discussed in terms of separate channels. In particular, a "basic access" digital pipe includes two B channels (basic channels) that each support 64 kbps signaling, and a D channel at 16 kbps. While the total bit rate of these three channels is 144 kbps, framing, synchronization and other overhead bits bring the total bit rate of a basic access link to 192 kbps. Furthermore, the B channels serve as separate communication channels such that the maximum data capacity, as viewed by the user, is 64 kbps per B channel, and 16 kbps for the D channel, not 192 kbps.
Conventionally, the function served by the ISDN D channel, is twofold. First, the D channel is used to establish and maintain signaling between the (customer provided equipment) CPE and the ISDN switch (operated by the telephone company). Thus, the D channel carries signaling information such as that required for dialing the telephone number of the destination terminal and making the connection between the source terminal and the destination terminal. A more complete description of the D channel as employed in narrowband and broadband ISDN, as well as ISDN terminal equipment, protocols, data rates, etc. is provided in the literature, for example in Stallings, W., "Data and Computer Communications", 5th Edition, Prentice Hall, 1997, pp 740-769 (hereinafter "Stallings") the contents of this book being incorporated herein by reference.
Other communication protocols are available as well for routing information from a source terminal to a destination terminal. These protocols include Frame Relay, Switch 56, asynchronous transfer mode (ATM), asynchronous digital subscriber line (ADSL), and digital subscriber line (DSL), which may serve as links to the source terminal's CPE and the destination terminal. A more complete description of Frame Relay and ATM protocols is provided in Stallings at page "301" to page "359".
FIG. 1 is a block diagram of a conventional ISDN-based system having a source facsimile 10 at a source facility 1 that communicates via an ISDN switch 22 to a destination facsimile 16 (or other type of destination terminal, such as a computer, ISDN equipped photocopier, etc.) in a destination facility 2. The destination facsimile 16 may, in turn, send the message to one or more subaddressees (Sub1, labeled as 16S1 in FIG. 1, to SubN, labeled as 16S2 in FIG. 1). The source facsimile 10 communicates via a terminal adapter 10A, shown as an internal device, although a separate external terminal adapter may be used as well. The terminal adapter 10A provides a protocol (physical layer and intermediate layer) conversion function for converting signal protocols such as V.35, RS-232, Universal Serial Bus (USB), IEEE 1394 (FireWire), etc. to an ISDN compliant protocol over a 4-wire interface.
The NT1 14 connects the source facilities 1, via a two-wire line 15, to a switching module 26 located at the ISDN switch 22. Alternatively, a second network termination (NT2) may be used at the source facility 1 between NT 1 and the terminal adapter 10A to provide a switching and concentration function, such as with a digital private branch exchange (PBX). Likewise, the NT1 may be replaced with a NT12 that performs the functions of both the NT 1 and NT2.
At the ISDN switch 22, the switching module 26 connects to a processor 24 and another switch module 28 via a bus 27, which allows digital commands and data to be passed between the respective switching modules 26 and 28, and the processor 24.
The equipment at the destination facility 2 may or may not be exactly similar to that of the source facilities 1. In the system shown at FIG. 1, the destination facility 2 is used as an example and includes the destination facsimile 16 having a terminal adapter 16A incorporated therein, which connects to another NT1 20 as shown. The subaddress systems 16S1 and 16S2 may be similarly configured, and are identified by respective subaddresses included in the message sent by the source facsimile 10. The NT1 20 connects to the switching module 28 in the ISDN switch 22, via another two-wire line 17 as shown. Thus the ISDN switch 22 connects to the source terminal 1 by a single communications link (line 15), and connects to the multiple subaddress systems 16S1 and 16S2 by another single communications link (line 17).
ISDN communications is based on a seven layer protocol stack, as explained in reference to FIG. A.5 of Stallings, for example. Control signaling is accomplished between the respective user-network interface and occurs at a third layer of the protocol stack (i.e., the "network"layer) and is named 1.451/Q.931. Thus, establishing and maintaining control signaling for a communication link established between the source facility 1 and the destination ISDN facility 2 and facility 3 is made through the D channel, and in particular, the ISDN network layer, data link layer and physical layer.
As appreciated by the present inventor, a user of the source terminal 1 can communicate to the separate subaddresses 16S1 to 16SN only if the link 17 is operational, and only if the NT1 20, terminal adapter 16A and destination facsimile 16 are operating properly, because these items are single points of failure for communicating to the separate subaddresses 16S1 to 16SN. Furthermore, the ISDN switch is not configured to send a message from the source facsimile 1 to multiple destination facilities 2, which are not connected by a common line 17. While, the ISDN may establish separate communication links to separate facilities, the establishment of these links is done on a per-request basis by the source facsimile, thereby requiring significant set-up time at the source facsimile 1, and incurring significant set-up cost.
FIG. 2 is a frame structure 200 of a transmission from the source facilities 1 to the ISDN switch 22, for an ISDN basic rate access. The frame structure 200 includes 48 bits that are transmitted in 250 .mu.sec. Components of the frame structure 200 include framing bits, F, dc balancing bits, L, B channel bits for the first B channel (16 per frame), B1, B channel bits for the second B channel (16 bits per frame), B2, D channel bits (4 per frame), D, auxiliary framing bit, Fa. A more detailed description of the frame structure, as well as a corresponding frame structure for the frames sent from the ISDN switch 22 to the source facilities 1, is described in Stallings, pp 212-215.
A link access protocol (LAPD) D channel is defined for establishing particular LAPD frames that are exchanged between the subscriber equipment (either at the source facility 1 or at the destination facility 2) and the ISDN switch 22. The call control protocol I.45 1/Q.931 is used on the D channel to establish, maintain and terminate connections on B channels.
A feature of the Q931 standard (section 5.1.1) is a call request operation where a setup message may employ "en-bloc" sending parameters, that among other things, establish a broadcast message that is received by the destination facility and distributed thereby to the respective subaddressees to which the broadcast message is to be distributed. In this case, the switch 22 does nothing to distribute the message from the source facility 1 to the separate subaddressees 16S1 to 16SN, but rather places the burden on the destination terminal 2 to administrate the broadcasting of the message to the respective subaddressees 16S1 to 16SN over separate lines 18S1 to 18SN (FIG. 1).
Tokyo Denki University Publishing Office published a paper entitled, "Illustrated Description for Technologies of ISDN Terminals" that describes the following "broadcasting" type incoming call procedure:
"In the broadcasting type incoming call procedure, the call is received by the network using the group TEI and the set-up message from a terminal is simultaneously transmitted towards all the addressed terminals and only terminals that are ready for communications respond to the call. When a plurality of the responses are simultaneously made, the network determines the order of a first-come first-served basis."
As appreciated by the present inventor, the above-described approach places the burden on the destination terminal 2 to simultaneously route the message to several subaddressees 16S1 to 16SN. The subaddresses are included in the Terminal endpoint identifier (TEI) portion of the ISDN setup message. This approach does not address the situation where a single message is sent from the source facility 1 to the switch 22, and have the switch 22 establish N different communication links (i.e., a 1.times.N switch operation) for communicating the message to N different destination terminals, which may be located at different locations (e.g., different states or countries).
FIG. 3 illustrates the signaling sequence between the source facility 1 and the ISDN switch 22. In order to establish each B channel connection between the source facility 1 and the destination facility 2, an initial communication link must be established on the D channel between the source facility 1 and the destination facility 2. To this end, a series of messages is sent back and forth between the source facilities 1 and the ISDN switch 22. This communication between the source facilities 1 and ISDN switch 22 occurs on a continuing basis on the D channel, while communications are maintained between the source facilities 1 and destination facilities 2 on the B channel. As shown in FIG. 3, several different messages are sent between the source facilities 1 and ISDN switch 22 while the D channel is maintained. One of the types of messages, as previously discussed is a broadcast message, where the destination terminal 2 is responsible for interpreting the message and routing the data message sent on the B channel(s) to the subaddressees 16S1 to 16SN.
The direction of the arrows in FIG. 3 indicates a direction of communication between the source facilities 1 and the ISDN switch 22. The process for establishing a connection is initiated by the source facilities 1 by first sending a setup message, which may include a broadcast message. Particular features of the setup message will be discussed with respect to FIG. 4, however the purpose of the setup message is to provide general information regarding the request to connect to the ISDN switch 22 and the destination facility 2. Next, the ISDN switch 22 responds with a call proceeding message that indicates that call establishment has been initiated. Subsequently, the ISDN switch 22, sends a connect message that indicates call acceptance by the source facilities 1 and destination facilities 2.
The source facilities 1 then sends a connect acknowledge signal that indicates the user has been awarded the call. When the user wishes to disconnect a call, the user sends a disconnect message via the source facilities 1 to the ISDN switch 22, requesting connection clearing. In response, a release message is sent from the ISDN switch 22, indicating the intent to release the channel and call reference. In response the source facilities 1 issues a release complete message, indicating that the release of the channel and the call reference. Subsequently, the call and information flow through the B channel is terminated.
FIG. 4 shows the structure of a conventional ISDN D channel setup message. The setup message includes respective LAPD frames (e.g., 501, 503 . . . ) of different sizes (measured in octets). The message includes a flag frame 501 that is one octet in length, followed by a service access point identifier (SAPI) frame 503 having a command/response bit (CR) and address field extension bit (0). The SAPI frame 503 is joined with the terminal end point identifier (TEI) frame 505, each of which are one octet in length. A control frame 507, is one or two octets in length, and is followed by an information frame 509, which has a variable length between 0 and 128 octets. A frame check sequence frame 511 follows and occupies two octets in length. An end frame 513 serves as an end of setup message flag.
The SAPI frame 503 includes a first subfield "SAPI", that identifies a protocol layer-3 user, as well as subframes C/R and 0, that are used as a predetermined formatting feature of SAPI. The TEI frame 505, is used to provide a unique terminal end point identifier that is used to identify the user's equipment, and in the case of a broadcast message, includes subaddresses of the subaddressees 16S1 to 16SN. The control frame 507 defines the type of frame format that will be employed such as an information frame, supervisory frame, and unnumbered frame for example. The information frame 509, includes a variable number of octets varying from 0 to 128 and contains respective subfields that contain any sequence of bits that form an integral number of octets.
Thus, when a user wishes to send data to a destination, information in the information field is passed directly to the destination user without the ISDN switch deciphering the contents of the information. Following the information field 509, the frame check sequence 511 is included and forms an error-detection function by calculating a code from the remaining bits of the frame, exclusive of the flags. The normal code is a cyclical redundancy check code. Finally, the end flag frame 513, includes a specific code indicating the end of the setup message.
As appreciated by the present inventor, a limitation with the broadcast approach to sending a message to several subaddressees 16S1 to 16SN, is that the subaddressees 16S1 to 16SN must be connected to a central routing mechanism (e.g., the destination facilities 2, in FIG. 1), where the burden is placed on the central routing mechanism to receive the message and simultaneously send copies to the respective subaddressees 16S1 to 16SN. Meanwhile, the ISDN switch 22 merely performs a rudimentary 1.times.1 switch function that receives one incoming message (over line 15 in FIG. 1) and sends one outgoing message over line 17 (FIG. 1).
The present inventor also recognizes that a limitation with conventional systems and methods is that users of a source terminal may wish to send a common message to many destination terminals that are not connected to one another, but all are connected to a switch. Thus, if a first destination terminal is located in Chicago and a second is located in San Francisco, and the first and second destination terminals are not directly connected to one another, the sender must send multiple transmissions in order to have the message sent to both destination terminals. This is both time inefficient and costly to the use of the source facility.
A related limitation with existing systems, as presently appreciated, is that conventional voice and data switches, such as those employed in Frame Relay, Switch 56, ATM, ADSL, and DSL do not support the ability to send a single message from a source facility and have the switch automatically establish N different communications links, where each communication link interconnects the source facility and the respective destination facility.